The present invention relates to a level discrimination device for a thumb wheel switch, which detects the respective levels of V.sub.DD (power source), V.sub.SS (GND) and FLOAT (insulation) of the thumb wheel switch to thereby detect the direction of rotation of the switch.
In a conventional thumb wheel switch, the direction of rotation is determined by detecting the phase difference between the output signals from, for example, two switches. FIG. 1 shows the timing charts of the signals, wherein .phi.1 is a reference signal and .phi.2 and .phi.2' are signals which are outputted upon rotation of the thumb wheel switch. The phase difference between the signal .phi.2 and the reference signal .phi.1 is detected, using a strobe signal .phi.B, and the phase difference between the signal .phi.2' and the reference signal .phi.1 is detected, using a strobe signal .phi.A. The signal .phi.2 is detected, when the switch has been rotated clockwise. The signal .phi.2' is detected, when the switch has been rotated counterclockwise.
Since the above-mentioned method uses strobe signals .phi.A and .phi.B to detect the direction in which the thumb wheel switch is rotated, the level discrimination device needs a timing circuit and its overall structure becomes complex. In addition, the method cannot detect the respective levels of the thumb wheel switch.